linux-kernel - Re: [PATCH v3 8/8] x86, mm, gup: prevent get_page() race with munmap in paravirt guest

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Date:   Mon, 16 Dec 2019 14:30:44 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ajay Kaher <akaher@...are.com>
Cc:     gregkh@...uxfoundation.org, stable@...r.kernel.org,
        torvalds@...ux-foundation.org, punit.agrawal@....com,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        willy@...radead.org, will.deacon@....com, mszeredi@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        srivatsab@...are.com, srivatsa@...il.mit.edu, amakhalov@...are.com,
        srinidhir@...are.com, bvikas@...are.com, anishs@...are.com,
        vsirnapalli@...are.com, srostedt@...are.com,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 8/8] x86, mm, gup: prevent get_page() race with munmap in paravirt guest

Peter Zijlstra <peterz@...radead.org> writes:

> On Tue, Dec 17, 2019 at 02:15:48AM +0530, Ajay Kaher wrote:
>> From: Vlastimil Babka <vbabka@...e.cz>
>> 
>> The x86 version of get_user_pages_fast() relies on disabled interrupts to
>> synchronize gup_pte_range() between gup_get_pte(ptep); and get_page() against
>> a parallel munmap. The munmap side nulls the pte, then flushes TLBs, then
>> releases the page. As TLB flush is done synchronously via IPI disabling
>> interrupts blocks the page release, and get_page(), which assumes existing
>> reference on page, is thus safe.
>> However when TLB flush is done by a hypercall, e.g. in a Xen PV guest, there is
>> no blocking thanks to disabled interrupts, and get_page() can succeed on a page
>> that was already freed or even reused.
>> 
>> We have recently seen this happen with our 4.4 and 4.12 based kernels, with
>> userspace (java) that exits a thread, where mm_release() performs a futex_wake()
>> on tsk->clear_child_tid, and another thread in parallel unmaps the page where
>> tsk->clear_child_tid points to. The spurious get_page() succeeds, but futex code
>> immediately releases the page again, while it's already on a freelist. Symptoms
>> include a bad page state warning, general protection faults acessing a poisoned
>> list prev/next pointer in the freelist, or free page pcplists of two cpus joined
>> together in a single list. Oscar has also reproduced this scenario, with a
>> patch inserting delays before the get_page() to make the race window larger.
>> 
>> Fix this by removing the dependency on TLB flush interrupts the same way as the
>
> This is suppsed to be fixed by:
>
> arch/x86/Kconfig:       select HAVE_RCU_TABLE_FREE              if PARAVIRT
>

Yes,

but HAVE_RCU_TABLE_FREE was enabled on x86 only in 4.14:

commit 9e52fc2b50de3a1c08b44f94c610fbe998c0031a
Author: Vitaly Kuznetsov <vkuznets@...hat.com>
Date:   Mon Aug 28 10:22:51 2017 +0200

    x86/mm: Enable RCU based page table freeing (CONFIG_HAVE_RCU_TABLE_FREE=y)

and, if I understood correctly, Ajay is suggesting the patch for older
stable kernels (4.9 and 4.4 I would guess).

-- 
Vitaly